Monday, June 14, 2010

Cardiac Arrest, from Ventricular Fibrillation.

The computerized QTc is 472, but the actual QTc is well over 600 ms. The T-waves are almost like a sine wave, and there are very prominent U-waves, best seen in precordial leads. These findings are pathognomonic of hypokalemia, and this was indeed the etiology of the cardiac arrest.


  1. Dr. Smith

    From this example, it would appear the QTc should be calculated from the end of the U wave, not T. Is that correct? If so, at what point (or amplitude) would you begin considering U waves as the end point for the QTc?

    Thank you for sharing your wisdom. New and old, your posts are enlightening.


  2. Good question! The answer is obscure, however. The computer is supposed to take the longest of the QT intervals as the final number. I don't know how it evaluates U-waves. If you look in precordial leads, the QT interval is not terribly long. If you look in limb leads, it is impossible to identify a U-wave: are the T- and U-waves merged? Is there no U-wave? Should the QT only be measured in the precordial leads where the T- and U-waves are distinct? I think these are academic questions; in practice, either the U-waves are very prominent, and this signifies probable hypokalemia, or there is a very long QT, which also signals hypokalemia.